FUNDAMENTAL FORMS OF CRYSTALS. 



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I. The first system includes the cube (fig. 1 or la, the lat- 

 ter in outline ;) regular octahedron (fig. 2;) and the rhombic 

 1 la 2 



dodecahedron (fig. 3 or 3a.) They are symmetrical solids 

 .hroughout, in all positions, being alike in having the height, 

 breadth and thickness equal ; their three axes, represented by 

 the dotted lines in the figures, are at right angles with one 

 another and equal.f »lifTiIie cube, the axes connect the cen- 

 ters of opposite faces \ in the octahedron and dodecahedron, 

 they connect the apice^s of solid angles. This is more fully 

 explained on a folio wink page. 



The cube has its factes equal squares, and its angles all 

 right angles. \ 



The octahedron has its 8 faces equal equilateral triangles : 

 its edges are equal ; its mane angles are 60° ; its interfacial 

 angles (angles between adjacent faces) 109° 28'. 



The dodecahedron hasiits 12 faces equal rhombs ; the j 

 edges are equal ; the plane angles of the faces are 109° 28' ; 

 and 70° 32' ; its interfacial angles are 120°. 



II. The second system Includes the right square prism 

 4 5 G 



(figs. 4 and 5,) and square octahedron (fig. 6.) The,y have 

 wo equal lateral axes, and la vertical axis unequal to the 



What forms docs the first system include 1 How are these forms 

 related % Describe the forms. What forme does the second system 

 aaclude, and how are they related ? Describe the forms. 



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